All audio systems are affected by the environment or room in which they are installed. For example, digital audio sources, such as compact discs and other types of discs (e.g., CDs and DVDs) have a +/−0.001 dB flat frequency response from 20 Hz to 20 kHz. Such audio sources also have a high S/N ratio of >100 dB, and negligibly low distortion levels of THD 0.001% at full scale. In addition, the digital signals are free from transient distortion, reverberation as well as ‘wow’ or ‘flutter’. However, when such high quality CDs or DVDs are played in a typical room, the room modifies the signal heard by the listener from what was originally intended. The speaker is responsible for some frequency deviation from the flat response and increased distortion but the room still has the largest affect on the audio quality.
A typical room can change a flat frequency response by greater than 40 dB. The highest affect is generally at the lower frequencies, such as below 300 Hz or more (i.e., Schroeder's frequency), when room modes are created. However, at higher frequencies reflections from walls, ceilings and floors cause not only frequency distortion but reverberation and in extreme cases a discrete echo can be heard.
The low frequency room modes can also cause very slow decay of sound notes which masks sounds near its frequency, which reduces the sound quality and intelligibility. As the effect is so dramatic on the audio, a number of attempts have been made to improve sound quality. A known conventional ‘solution’ is to adjust the room dimensions such that the height to width and height to length ratio is not an integer. However, this is not possible if the room has already been designed. Other conventional solutions may be to treat the room with sound absorbers, baffles and bass traps as is done in recording studios. However, this can be very expensive to do or may not be viable when the room is a conference room or a room used for multiple purposes or living in general.
The earliest attempts at room correction used graphic equalizers. The most sophisticated graphic equalizers were ⅓ octave (33-bands). As the quality (Q) for ⅓ octave is only 4.3 this Q is clearly not high enough to correct the room modes. Also, the frequency overlapping nature of the 33-band graphic equalizer makes it difficult to dial-in a correction. Later DSP based attempts at room correction involved inverting the room response. This approach would clearly require a huge processing task as the room response of a large room can be greater than 1 second (48000 samples at 48 kHz sampling frequency). However, none of these early attempts have successfully optimized sound quality. In-fact, such audio conventional correction efforts have even worsened the sound quality in certain circumstances.
Most if not all room equalization systems design a black box correction system. For example, once the filters have been calculated, there is no user intervention. To the contrary, example embodiments of the present application allow for customized system design, which allows infinite user changes to the filters designed.